Ann Merchant Boesgaard

Chemical composition of open clusters. I. Fe/H from high-resolution spectroscopy. II. C/H and C/Fe in F dwarfs from high-resolution spectroscopy

Using high-resolution spectroscopy, the abundance ratios Fe/H, C/H, and C/Fe were determined for F dwarfs in the Alpha Per, the Pleiades, and the Hyades clusters; the UMa, Hyades, and Wolf 630 moving groups; and a selection of bright F field dwarfs. The age span of these objects ranges from 5 x 10 to the 7th to 2 x 10 to the 9th yr. No evidence was found of a trend in Fe/H with age for these clusters and groups, but there were clear differences in Fe/H among these groups, indicating intrinsic differences in the metal content of the local gas out of which these groups were formed. No evidence was found for a trend of C/H with age of these stellar groups, but there were cluster-to-cluster variations, implying differences in the content of carbon in the precluster gas. The C/H cluster differences followed the same pattern as the Fe/H cluster differences and yield C/Fe values which are constant, and equal to the solar value, in all the groups. 53 refs.

Beryllium Abundances in Halo Stars from Keck/HIRES Observations

We have determined the abundance of Be in stars with an array of metal abundances in order to enhance our understanding of the chemical evolution of the Galaxy, cosmic-ray theory, and cosmology. Observations of the Be II resonance lines at j3130 and j3131 were made at the Keck telescope with the HIRES spectrometer at a resolution of 46,000 and signal-to-noise ratios of 60¨110 (per pixel) typically. Our sample includes 22 halo dwarfs and —ve disk stars (including the Sun). We have taken special care in determining the stellar parameters for these stars in a consistent manner. The Be abundances were found (1) from the measured equivalent width of the relatively unblended Be II line at 3131.065 with Ae an analysis that included 11 weak atomic and molecular lines near that wavelength and (2) from spec- trum synthesis that included newly derived enhanced O (relative to Fe) in the synthesis calculations. The two methods are in excellent agreement. We —nd straight-line —ts between Be and Fe: log N(Be/H) \ 0.96(^0.04)(Fe/H) ( 10.59(^0.03) ; and between Be and O: log N(Be/H) \ 1.45(^0.04)(O/H) ( 10.69(^0.04) . It seems that Be and Fe increase at the same rate during the course of the evolution of the Galaxy. But as O increases by a factor of 100, Be increases more rapidly, by a factor of 800. Traditional models in which energetic cosmic rays interact with ambient CNO nuclei in the interstellar medium to produce Be are consistent with this —nding, as long as certain chemical evolution eUects (such as mass out—ow from the halo) are taken into account. However, models predicting a linear relationship between Be and O, such as those producing Be in the vicinity of Type II supernovae, are less consistent with our result. There is some evidence for an intrinsic spread in Be at a given (Fe/H) or (O/H). There is currently no evidence of a primordial plateau level of Be down to log N(Be/H) \( 13.5.

Chemical composition of open clusters. III: Iron and carbon in F dwarfs in Coma, Praesepe, and M67

Iron and carbon abundances have been determined for F dwarfs in several open clusters and the field to continue our investigations on the homogeneity of chemical mixing in the Galactic disk. These new data on stars in Coma, Praesepe, M67, and the field complement our previous sample of clusters and moving groups and extend the sample coverage in metallicity and age. The data are high resolution, high signal-to-noise Reticon observations made with the Canada-France-Hawaii Telescope coude

The Evolution of Galactic Boron and the Production Site of the Light Elements

The Goddard High Resolution Spectrograph (GHRS) of the Hubble Space Telescope (HST) has been used to obtain spectra of the 2500 A region in eight stars with metallicities ranging from [Fe/H] = -0.4 to -3.0, including the most metal-poor star ever observed for boron. Spectrum synthesis utilizing latest Kurucz model atmospheres has been used to determine the B abundance for each star, with particular attention paid to the errors of each point, to permit judgment of the quality of the fit of models of Galactic chemical evolution. Previous observations were combined with new ones, bringing the number of stars analyzed to 11. A straight line of slope ≈ 1 gives an excellent fit to a plot of log (BLTE) versus [Fe/H], and if NLTE B abundances are used, the slope is ≈ 0.7. Plotting B versus [O/H] rather than [Fe/H] increases the slope of either plot by about 0.2. The observed relation suggests that the production of light elements such as B and Be is directly related to the production of heavier elements. Our data do not show a change in slope between halo and disk metallicities, but the number of stars near the disk-halo transition is small, and a modest change is not precluded. The NLTE B/Be ratio is typically ≈ 15 throughout the lifetime of the Galaxy, a ratio naturally produced by cosmic-ray (CR) spallation. Our data support a model in which most light-element production comes from low-energy CR spallation of C and O nuclei onto protons and α-particles, probably in the vicinity of massive supernovae in star-forming regions. Until recently, most models have emphasized light-element production in the general ISM from the spallation of high-energy protons and α-particles onto CNO nuclei. Especially during the Galaxys early history, when the metallicity of the ISM was low, the spallation of protons and α-particles onto CNO nuclei cannot account for as much B as we observe, unless the CR flux was sufficiently high for compensation. The observed relation also constrains any direct production of B by the ν-process in supernovae to be at most a small part of total B production. It is possible that the gamma rays recently detected from the Orion Nebula region are the signature of spallation by energetic C and O nuclei. Nevertheless, B, Be, and Fe data alone give the strongest evidence of the importance of spallation by C and O for producing light elements.

Lithium in the Hyades, the Hyades moving group, and Praesepe

High-resolution spectra of 35 F main-sequence stars have been obtained at high SNRs in the region of the Li I resonance line at 6708 A. Fourteen of the stars are members of the Hyades, six are members of Praesepe, and the remaining 15 stars are members of the Hyades Moving Group. Equivalent widths of the Li resonance line and of Fe lines in the same spectral region were measured and Li and Fe abundances were calculated from spectral synthesis using Kurucz model atmospheres. A mean metallicity of (Fe/H) = + 0.17 + or - 0.06 for the Hyades, + 0.13 + or - 0.07 for Praesepe, and + 0.11 + or - 0.09 for the Hyades Moving Group is obtained. The Li abundance patterns are consistent with those found previously for the Hyades stars and for the Praesepe stars, but some of the Hyades Moving Group members do not fit this Li-temperature profile well. The possibility exists that the Hyades Moving Group is not coeval and that its members have little in common other than kinematics. 29 references.

The evolution of chromospheric activity and the spin-down of solar-type stars

IUE data for 31 solar-type stars are compared with observations of T Tauri stars in order to determine whether the pattern of main sequence chromospheric decay shown by stars older than about 100 million yr extends back to ages of 100 million yr appropriate for T Tauri stars. An analysis of the time decay of stellar ultraviolet and X-ray emission establishes a relationship between emission level and axial rotation, which is expressed in terms of the Rossby parameter. It is shown that the intensity of ultraviolet chromospheric and transition region lines of solar-type stars declines with age. The activity-age relation for main sequence stars older than about 100 million yr is fitted best by an exponential law whose rate of falloff with age depends on temperature.

Beryllium in the Hyades F and G Dwarfs from Keck HIRES Spectra

Although there are extensive observations of Li in field stars of all types and in both open and (recently) globular cluster stars, there are relatively few observations of Be. Because Be is not destroyed as easily as Li, the abundances of Li and Be together can tell us more about the internal physical processes in stars than either element can alone. We have obtained high-resolution (45,000) and high signal-to-noise ratio (typically 90 per pixel) spectra of the Be II resonance lines in 34 Hyades F and G dwarfs with the Keck I telescope and HIRES. In addition we took a spectrum of the daytime sky to use as a surrogate for the solar spectrum so we could determine the value for Be in the Sun, analyzed in the same manner as that for the stars. We have adopted stellar temperatures and some Li abundances for these stars from the literature. For most of the F dwarfs we have rederived Li abundances. The Be abundances have been derived with the spectrum synthesis method. We find that Be is depleted, but detected, in the Li gap in the F stars reaching down to values of A(Be) = 0.60 dex, or a factor of nearly 7 below the meteoritic Be abundance (a factor of 3.5 below the solar value of Chmielewski et al.). There is little or no depletion of Be in stars cooler than 6000 K, in spite of the large depletions (0.5-2.5 dex) in Li. The mean value of A(Be) for the 10 coolest stars is 1.33 ± 0.06, not far from the meteoritic value of 1.42. The pattern in the Be abundances—a Be dip and undepleted Be in the cool stars—is well matched by the predictions of slow mixing due to stellar rotation. We have interpolated the calculations of Deliyannis and Pinsonneault for Be depletion due to rotational mixing to the age of the Hyades; we find excellent agreement of the predictions with the observed Be abundances but less good agreement with the observed Li abundances. Some of our Hyades stars have photometrically determined rotation periods, but there is no relation between Be and rotation period. (Generally, the lower mass stars have less Li and longer periods, which may indicate greater spin-down and thus more Li depletion relative to Be.) The Li and Be abundances are correlated for stars in the temperature range of 5850-6680 K, similar to results from earlier work on Li and Be in F and G field stars. This indicates that the depletions are not just correlated—the only claim that can be made for the field stars—but are probably occurring together during main-sequence evolution. The Hyades G dwarfs have more Be than the Sun; their initial Be may have been larger or they may not be old enough to have depleted much Be. For those Hyades stars that seem to have little or no depletion of Li or Be, the Li/Be ratio is found to be 75 ± 30; the meteoritic ratio Li/Be is 78. The Hyades ratio is a representative value for the initial ratio in the material out of which the Hyades cluster was formed.

Galactic evolution of beryllium

The abundance of Be in the lowest-metallicity stars is a probe of Big Bang Nucleosynthesis, and its abundance in halo and disk stars is a probe of galactic evolution and stellar structure. We present observations of the Be II resonance lines in 14 halo stars and 27 (mostly old) disk stars with [Fe/H] from −2.7 to +0.13. The spectra were obtained at the Canada-France-Hawaii 3.6 m telescope and have a measured resolution of 0.13 A and a median signal-to-noise ratio of ∼50. For 18 of the 41 stars we have also made observations of the O I triplet at the Palomar 5 m telescope, the UH 2.2 m telescope, and the CFH telescope. Stellar parameters of T eff , log g, and [Fe/H] were carefully determined from several independent estimates

Primordial Lithium: Keck Observations in M92 Turnoff Stars

We present new Keck I/HIRES observations at R = 45,000 (=3 pixels) of seven stars near the turnoff of the old, metal-poor globular cluster M92. In three of these stars, we have signal-to-noise ratios (S/Ns) of 40 pixel-1, and in the other four, the S/N is near 20. The Li abundance in star 18 is high compared with the halo field-star plateau and is similar to that in the remarkable Li-rich halo field star BD +23°3912. In addition to the high Li abundance in star 18, there is a dispersion in Li abundance in our seven stars covering the full range of a factor of 3. We have attempted to determine whether the excess Li in star 18 is due to less than average Li depletion in this star from an even higher initial abundance, as predicted by the Yale rotational models, or whether it is due to the extraordinary action of Li production mechanisms in the material that formed this star. We have found no convincing evidence that favors Li production: (1) Stars 18, 21, and 46 have identical Ba abundances, which argues against Li production carrying an s-process signature. (2) These three stars have indistinguishable Ca, Cr, Fe, and Ti, which argues against supernova Li production. (3) We discuss ν-process production of Li and find no convincing observational evidence for this from the strengths of the Mg, Ca, and Fe lines. (4) The similarity in age of these cluster stars argues against cosmic-ray Li production that requires age differences of gigayears. The most likely explanation for the Li dispersion is differential Li depletion from a (possibly significantly) higher primordial Li abundance due to differences in the initial angular momentum in each star followed by spin-down; the most rapid rotators destroy the most Li, whereas the initially slower rotators preserve more Li.

Keck HIRES Spectroscopy of M92 Subgiants: Surprising Abundances near the Turnoff

Using high-resolution, moderate signal-to-noise ratio spectroscopy obtained with the 10 m Keck I Telescope and efficient HIRES echelle spectrograph, we derive abundances of several elements in sub- giants near the M92 turno†. As a consistency check, we also analyze the metal-poor -eld star HD 140283 and -nd an Fe abundance in -ne agreement with many previous determinations. However, our M92 value ((Fe/H) \( 2.52) is a factor of 2 lower than the abundance derived from its red giant members. Di†erences in model atmospheres, gf-values, and instrumental e†ects might account for this di†erence, but whether they in fact do so is unclear. We note possible evidence for (Fe/H) di†erences within M92. Our spectroscopic analysis suggests that the M92 reddening, E(B(V ), may be 0.04E0.05 mag greater than canonical values, but various uncertainties mean that this conclusion is not de-nitive; the signi-cant di†erence in interstellar Na I line strengths in the M92 and HD 140283 spectra may be consistent with an increased reddening. Regardless, the conclusion that either the (Fe/H) of M92 has been signi-cantly overestimated from red giants or current reddening/photometry estimates are too small/red is not easily escaped. If the reddening/photometry were in error by this amount, turno† colorE based ages for M92 could be reduced by D4 Gyr. The adjustment to the M92 distance modulus required for a similarly reduced turno† age that is luminosity-based can be accommodated by increases in extinction and alterations to the metal-poor -eld star distance scale recently inferred from Hipparcos Cepheid and subdwarf data. Our M92 subgiants demonstrate (Cr/Fe), (Ca/Fe), and (Ti/Fe) ratios that are unremarkable and essentially identical to the values for HD 140283. (Ba/Fe) is 0.45 dex larger for the M92 subgiants than for HD 140283. Surprisingly, we -nd (Mg/Fe) to be 0.55 dex lower in our M92 subgiants than in HD 140283, and (Na/Fe) to be 0.76 dex larger in our M92 subgiants than in HD 140283. These di†erences (and indeed nearly all our abundance ratios) seem immune to various data, analysis, and parameter errors. If real, this striking abundance pattern is suggestive of material in our M92 starsI photospheres that has undergone Ne ) Na and Mg ) Al cycling like that inferred for red giants in M92 and other clusters. While this is generally believed to be an in situ process in cluster giants, the presence of abun- dant Li in our M92 objects suggests a polluting source acting either primordially or via accretion after cluster star formation. This may be consistent with CN and Na variations on the 47 Tucanae main sequence, recently reported Ba and Eu variations in M15 red giants, possible cluster-to-cluster n-capture abundance di†erences, and very low (O/Fe) ratios observed near the base of the M13 giant branch. We thus suggest that a polluting source of light-element alteration, in addition to the in situ source for more evolved stars, may be required for M92. Comparison of our M92 subgiant abundance ratios with those of M92 red giants may indicate that pollution occurred after the present generation of cluster stars formed, but until the cause or causes of the subgiant versus giant Fe abundance discrepancy are de-ni- tively identi-ed, this conclusion is uncertain. A polluting source of our Na and Mg anomalies produced via processing in a previous stellar generation also has complications; namely, how the Mg and Na anomalies arise without apparently any net inNuence on our subgiantsI Li abundances and on the C abundances of other M92 subgiants. A similar quandary may exist in some 47 Tuc turno† stars. An understanding of cluster abundance variations (by whatever mechanisms) and their behavior with evolu- tionary state may be needed for a complete understanding of absolute and relative globular clusters ages, and for derivation of the primordial Li abundance.